This application claims priority under 35 U.S.C. xc2xa7119 to Great Britain Patent Application No. 0021748.9 filed Sep. 2, 2000.
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1. Field of the Invention
The present invention relates to a transmission chain and more particularly to an inverted tooth chain that is used typically as a timing chain in an internal combustion engine of a vehicle.
2. Description of the Related Art
An inverted tooth timing chain comprises a plurality of links interconnected by pins. The links comprise link plates at least some of which are connected such that they articulate on the pins. The link plates have depending teeth that mate with the teeth on the periphery of drive and driven sprockets.
The timing chain passes around a drive sprocket attached to the crankshaft of an internal combustion engine and around a driven sprocket that is attached to the camshaft so that rotary motion of the crankshaft is transmitted synchronously to the camshaft. The synchronous rotation of the sprockets is important to ensure that the movement of valves on the camshaft is in an accurate timed relationship with the movement of the pistons on the crankshaft.
In use, the individual link plates of such chains are subjected to rubbing contact with other link plates or the pins of the chain and are therefore prone to wear. Wear in chains leads to chain elongation, inefficient power transmission or unmeshing of the chain from the sprockets. This is particularly undesirable in timing chains as the camshaft may rotate several degrees out of alignment with the crankshaft and render the engine inefficient or inoperative.
It is well-known to subject the components of chains to heat treatment to improve chain endurance and to reduce the tendency to wear. The link plates and rollers may be austempered or quenched and the bushes carburized. The pins are typically carburized or coated with a diffusion alloy. These treatments are well documented in the art and are known to increase the surface hardness of the components while reducing their tendency to wear.
U.S. Pat. No. 5,437,148 describes a chain in which the link plates are coated by a dry permanent lubricant such as MoS2. Similarly, JP 49 106449A discloses the coating of chains with MoS2 by electro-disposition of the Molybdenum layer and then sulphiding.
It is an object of the present invention to provide for a transmission chain in which the resistance to wear is improved.
According to a first aspect of the present invention there is provided a transmission chain having a plurality of links interconnected by pins, the links comprising link plates that have a surface layer of iron bisulphide.
The surface layer of iron bisulphide produces the best results on inverted tooth timing chains but can also be used in relation to other transmission chain with improved results.
The chain links may further comprise rollers or bushes which have a surface layer of iron bisulphide.
In a preferred embodiment the iron bisulphide layer has a thickness of between 4 and 15 xcexcm.
According to a second aspect of the present invention there is provided an inverted tooth transmission chain having a plurality of link plates interconnected by pins, the link plates having a surface layer of iron bisulphide.
According to a third aspect of the present invention there is provided a method for reducing the initial wear rate and improving the overall wear resistance of a inverted tooth transmission chain of ferrous material comprising placing link plates of the chain in a sulphur-rich atmosphere so as to form a surface layer of iron bisulphide thereon and assembling the chain by interconnecting the link plates with pins.